Ultrasound applying skin care device

ABSTRACT

An ultrasonic skin care device has an applicator head for applying the ultrasound vibrations to a user&#39;s skin. The applicator head has a vibrator element and a horn which are integrated into a combined vibration mass that resonates with an electric pulse to produce the ultrasound vibrations. The device is configured to limit the ultrasound upon detection that the applicator head is out of a normal contact with the skin. A load detecting circuit detects whether the applicator head is in a normally loaded condition or in an unloaded condition with reference to electrically equivalent impedance of the combined vibration mass. The combined vibration mass has a structure that restrains vibrations at a center portion of the vibration mass in order to reduce an undesired parasitic resonance, thereby enabling to discriminate the impedance given under the normally loaded condition from that given under the unloaded condition.

TECHNICAL FIELD

The present invention is directed to an ultrasound applying skin caredevice for applying an ultrasound to a user's skin in order toaccelerate the metabolism of a skin tissue for making the skin beautyand healthy.

BACKGROUND ART

WO98/51255 discloses a like ultrasound applying skin care device havingan applicator head for generating and transmitting an ultrasound to auser's skin. The device includes a load detecting circuit for detectionwhether the applicator head is loaded by contact with the skin in orderto save energy when the applicator head is out of contact with the skinas well as to apply the ultrasound safely when the applicator head is incontact with the skin. For judging whether the applicator head is loadedor unloaded, the device relies on an electrically equivalent impedanceof the applicator head which varies with the load acting on theapplicator head, and compares a corresponding voltage being applied tothe applicator head with a reference voltage. However, when theapplicator head gives the ultrasound at a relatively high frequency, forexample, several MHz or more, it may be sometimes difficult todiscriminate the unloaded condition from the loaded condition on a basisof the electrically equivalent impedance due to an increased effect of aparasitic resonance appearing in the applicator head.

Japanese Patent Publication No. 7-59197 discloses an ultrasonic vibratorelement that reduces vibrations occurring around the periphery of thevibrator element. The vibrator element is in the form of a circular diskprovided on opposite surfaces thereof respectively with upper and lowercircular electrodes. Each of the electrodes has a diameter less thanthat of the vibrator element to leave the outer periphery of thevibrator element uncovered in an attempt to cancel undesired vibrationspropagating in radial directions, thereby allowing intended ultrasonicvibrations to proceed in the thickness direction of the vibratorelement.

The parasitic resonance may be reduced with the use of the structuredisclosed in the Japanese Patent Publication No. 7-59197 to some extent.After studying the behavior of the ultrasonic vibrations at the highfrequency, the inventors have found effective to restrain vibrationsoccurring at the center of the applicator head in order to reduceundesired parasitic resonance to such an extent that the applicator headcan exhibit the electrically equivalent impedances, respectively whenbeing loaded and unloaded, that are sufficiently distinct from eachother for easy discrimination between the loaded and unloaded condition.

DISCLOSURE OF THE INVENTION

In view of the above finding, the present invention has beenaccomplished to provide an ultrasound applying skin care device which iscapable of applying the ultrasound effectively and safely for enhancingthe skin care.

The skin care device of the present invention includes a housingprovided with an applicator head for applying the ultrasound to a user'sskin, and a driver circuit that provides an electric pulse for actuatingthe applicator head to transmit the ultrasound to the skin. Theapplicator head is composed of a vibrator element generating theultrasound, and a horn having a mounting face and a skin opposing facefor use in contact with the skin. The horn carries the vibrator elementon the mounting face to transmit the ultrasound to the skin. Thevibrator element and the horn are integrated into a combined vibrationmass that resonates with the electric pulse from the driver circuit,thereby transmitting resulting vibrations to the skin. The combinedvibration mass gives a first electrically equivalent impedance when itis normally loaded by contact with the skin, and gives a secondelectrically equivalent impedance when it is unloaded. The deviceincludes a load detecting circuit which monitors whether the combinedvibration mass give the first or second electrically equivalentimpedance and provides a load detection signal only upon seeing thefirst electrically equivalent impedance. Also included in the device isa control circuit which limits or stops the electric pulse when the loaddetection signal is not received within a predetermined time period. Thefeature of the present invention resides in that the combined vibrationmass has a structure that restrains vibrations at a center portion ofthe vibration mass in order to reduce a parasitic resonance, therebydifferentiating the first electrically equivalent impedance from thesecond electrically equivalent impedance. Thus, the load detectingcircuit can successfully judge whether the applicator head is in contactwith or out of contact from the skin, whereby the control circuit can bemade reliable to limit the ultrasonic vibrations from being generatedwhen the applicator head is unloaded.

Preferably, the vibrator element is composed of a piezoelectric elementin the form of a circular disc having flat upper and lower end facesprovided respectively with upper and lower electrodes across which theelectric pulse is applied. At least one of the upper electrode, thelower electrode, and the piezoelectric element has a center openingwhich is responsible for restraining the vibrations at the center of thecombined vibration mass.

In addition to the center opening, at least one of the upper and lowerelectrodes may be dimensioned to have a diameter smaller than that ofthe piezoelectric element to leave the peripheral portion of thepiezoelectric element uncovered also for reducing undesired vibrationsaround the periphery of the piezoelectric element.

Alternatively, at least one of the upper and lower electrodes is dividedby at least one slit into a plurality of identical segments. The slitextends diametrically to leave the center and the diametricallyextending band portion of the piezoelectric element uncovered forrestraining the vibrations at the center of the vibration mass.

Instead of providing the diametrically extending slit, at least one ofthe upper and lower electrodes may be configured to have at least oneslit that uncovers the center portion of the piezoelectric element forthe same purpose of restraining the vibrations at the center of thevibration mass.

In combination with or separately from the provision of the centeropening in at least one of the upper electrode, the lower electrode, andthe piezoelectric element, the horn may be configured to have a centerhole in the form of a through-hole or cavity for restraining thevibrations at the center of the vibration mass.

Further, instead of being formed with the center opening, the upperelectrode may be covered with an elastic member that absorbs thevibrations at the center portion of the vibration mass for reducing theparasitic resonance.

Still further, the upper electrode may be covered on its center with asolder bulk for electrical connection of the upper electrode to a leadwire leading from the driver circuit. The solder bulk adds a weight tothe center of the combined vibration mass for restraining the vibrationsat the center portion thereof.

Further, the horn is preferred to be formed as an integral part thereofwith a rim which surrounds the horn and which is adapted to secure thehorn to the housing. Defined between the horn and the rim is arestrictor which restricts the ultrasound from propagating towards therim, thereby concentrating the ultrasound to the horn for effectivelytransmitting the ultrasound to the skin through the horn. The restrictormay be in the form of a cavity formed along the boundary between thehorn and the rim.

Preferably, the control circuit is designed to receive the firstelectrically equivalent impedance in order to vary the intensity of theultrasound generated at the vibrator element in accordance with themagnitude of the first electrically equivalent impedance. As the firstelectrically equivalent impedance will vary depending upon a pressure atwhich the horn or the combined vibration mass is held against the user'sskin, the device can vary the effect or the strength of the ultrasoundbeing applied to the skin depending upon the pressure, thereby applyingthe ultrasound optimally to the user's skin for enhanced skin careresult.

Still further, the device is preferred to include a motion detectingcircuit which monitors whether the combined vibration mass is moving andprovides a motion detection signal when the vibration mass is so moving.The control circuit is connected to receive the motion detection signaland controls the driver circuit to stop or limit the electric pulse whenthe motion detection signal is not continuous over a critical timeduration even in the presence of the load detection signal beingdetected within the predetermined time period.

These and still other objects and advantageous features of the presentinvention will become more apparent from the following detaileddescription of the preferred embodiment when taken in conjunction withthe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front sectional view of an ultrasound applying skin caredevice in accordance with a preferred embodiment of the presentinvention;

FIGS. 2A to 2C show improper use conditions of above device;

FIG. 3 is a block diagram of an electric circuitry of the above device;

FIG. 4 is a circuit diagram illustrating a driver circuit, a loaddetecting circuit, a motion detecting circuit of the above circuitry;

FIGS. 5A to 5F are waveform charts illustrating the operation of theload detecting circuit and the motion detecting circuit;

FIG. 6 is a circuit diagram illustrating a temperature sensing circuitof the above circuitry;

FIG. 7 is a flow chart illustrating the operations of the device;

FIG. 8 is a top plan view of an applicator head of the above device;

FIG. 9 is a sectional view of the applicator head;

FIGS. 10 to 12 are partial views illustrating modified structure of theapplicator head;

FIG. 13 is a schematic view illustrating a relation between thewavelength of the ultrasound of different frequencies and a combinedvibration mass of the applicator head;

FIG. 14 is a graph illustrating, as a comparative purpose, electricallyequivalent impedances of the vibration mass given under a normallyloaded condition, an unloaded condition, and an abnormally loadedcondition, respectively, without a structure of reducing a parasiticresonance;

FIG. 15 is a top plan view of the vibrator element;

FIG. 16 is a sectional view of the vibrator element;

FIG. 17 is a graph illustrating the electrically equivalent impedancesof the vibration mass given under the normally loaded condition, theunloaded condition, and the abnormally loaded condition, respectively,for the combined vibration mass in accordance with the embodiment of thepresent invention;

FIGS. 18 and 19 are top and sectional views of the vibrator element inaccordance with a modification of the above embodiment;

FIGS. 20 and 21 are top and sectional views of the vibrator element inaccordance with another modification of the above embodiment;

FIGS. 22 and 23 are top views illustrating further modifications of theabove embodiment;

FIGS. 24 and 25 are top and sectional views of the vibrator element inaccordance with a still further modification of the above embodiment;

FIGS. 26 and 27 are top view illustrating further modifications of theabove embodiment;

FIGS. 28 and 29 are top and sectional views of the vibrator element inaccordance with a more modification of the above embodiment;

FIGS. 30 and 31 are sectional views illustrating modifications of theabove embodiment;

FIGS. 32 and 33 are top and sectional views of the vibrator element inaccordance with a further modification of the above embodiment;

FIGS. 34 and 35 are top and sectional views of the vibrator element inaccordance with a more modification of the above embodiment;

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates an ultrasound applying skin care device in accordancewith a preferred embodiment of the present invention. The skin caredevice is utilized for face care or skin massage for enhancing themetabolism of a skin tissue for making the skin beauty and healthy. Thedevice includes a hand-held grip housing 10 provided at its one endthereof with an applicator head 100 which is adapted in use to contactwith a user's skin for applying ultrasound thereto. The applicator head100 is composed of a vibrator element 110 in the form of a piezoelectricelement generating the ultrasound, and a horn 120 transmitting theultrasound to the skin S. The piezoelectric element is shaped into acircular disc having a flat upper surface and a flat lower surface whichare covered respectively with upper and lower electrodes 111 and 112across which an electric pulse is applied for generating the ultrasoundvibration. The vibrator element 110 and the horn 120 are integrated intoa combined vibration mass M which is caused by the electric pulse toresonate for generating and applying the resonant ultrasound vibrationto the skin S. Preferably, the device is designed to generate theultrasound having a frequency of 1 MHz to 10 MHz and transmitted to theskin at an intensity of 0.1 W/cm² to 2.0 W/cm². Further, it is preferredto use a gel or like fluid F at the interface between the horn and theskin for promoting the transmission of the ultrasound to the skin S.

As will be explained later in details, the device is equipped with asafe-guard for limiting or stopping the ultrasonic vibrations beingtransmitted to the skin when the applicator head 100 is not in anormally loaded condition of FIG. 1, i.e., the applicator head is heldin any one of improper conditions. As shown in FIGS. 2A to 2C, theimproper conditions include an unloaded condition where the applicatorhead 100 is away from the skin (FIG. 2A), a partial contact conditionwhere the applicator head 100 is placed only partially against the skin(FIG. 2B), and a direct contact condition where the applicator head 100is placed against the skin without using the fluid F at the interfacetherebetween (FIG. 2C).

As shown in FIG. 3, the device includes a driver circuit 20 forproviding the electric pulse across the electrodes 111 and 112 of thepiezoelectric element 110, a load detecting circuit 40 for detection aload condition of the applicator head 100, a motion detecting circuit 50for detection of a motion of the applicator head 100, a temperaturesensing circuit 60 for sensing a temperature of the piezoelectricelement 110, a display driver circuit 170 for displaying operatingconditions of the device, and a control circuit 80 for control of theabove circuits. The driver circuit 20 is energized by a power supply 1accommodated within a separate power pack 2 for converting a commercialAC line voltage into a DC voltage. Also included in the device is amonitoring circuit 90 for monitoring the ultrasound being generated andapplied to the user's skin based upon an electrically equivalentimpedance of the combined vibration mass M.

The device 10 is designed to generate the ultrasound while the horn 120is kept substantially in contact with the use's skin. For this purpose,the load detecting circuit 40 is provided to detect whether a suitableload is applied as a consequence of the horn 120 being in contact withthe user's skin via the fluid F. When the horn 120 is not in contactwith the skin and fails to transmit the ultrasound successfully, theload detecting circuit 40 determines that the horn 120 or the vibrationmass M is not loaded and restricts the generation of the ultrasound. Thedetails of the load detection realized in the present invention will bediscussed later. In use, it is desirable to move the applicator head100, i.e., the combination mass slowly across the skin when applying theultrasound. Otherwise, when the applicator head 100 stays at a portionover a long period, there is a potential hazard of causing a cold burnin the skin. In view of this, the motion detecting circuit 50 isprovided to enable the continuous ultrasound application when theapplicator head 100 is moving at a suitable rate and otherwise disableor limit the ultrasound generation. In addition, the control circuit 80includes a timer which stops generating the ultrasound after the deviceis utilized over a preset time. That is, the timer will count a timeonly when the load detection signal from the load detecting circuit 40indicates that the applicator head 100 is kept in the normal contactwith the skin and when the motion detection signal from the motiondetecting circuit 90 indicates that the applicator head 100 does notstay at a portion over a long time. The timer operates to continuegenerating the ultrasound over the preset time. Also, after the presettime is elapsed, the control circuit 80 gives an instruction to stopproviding the electric power to the driver circuit 20, ceasing theultrasound generation.

When the vibration mass suffers from abnormal vibrations with anattendant temperature rise due to malfunction of the driver circuit 20or the like, the temperature sensing circuit 60 is responsive to anoutput from a temperature sensor 15 located adjacent the horn 120 forproviding an output indicative of abnormal temperature rise to thecontrol circuit 80 which in turn responds to stop the driver circuit 20.

As shown in FIG. 4, the driver circuit 20 includes an inverter whichconverts DC voltage from the power supply 1 into an AC voltage. Providedat the output end of the inverter is a transformer T with a primarywinding 21 and a secondary winding 22. The primary winding 21 isconnected in series with FET 23 and a current sensing resistor 27 acrossthe power supply 1, and is cooperative with a capacitor 24 to form aparallel resonant circuit which provides a resonant voltage across theprimary winding 21 upon turning off of FET 23. The piezoelectric element110 is connected across the secondary winding 22 so as to effect theultrasound vibrations by the AC voltage or the electric pulse induced atthe secondary winding 22. A feedback winding 25 is coupled to theprimary winding 21 to feedback the output of the driver circuit to FET23. A bipolar transistor 26 is connected in a gate-emitter path of FET23 for control of FET 23. Connected across the power supply 1 is aseries combination of a starting resistor 28 and a capacitor 29 of whichconnection is connected through the feedback winding 25 to a gate of FET23 to give a bias thereto. When capacitor 29 is charged by the powersupply 1 to develop a voltage reaching a threshold of FET 23, FETbecomes conductive to lower the drain voltage of FET 23. At this time,the feedback winding 25 generates a feedback voltage applied to the gateof FET 23, thereby increasing the current flowing through the FET.Subsequently when a voltage developed across current sensing resistor 27reaches a predetermined level in correspondence to the increasingcurrent through FET, transistor 26 becomes conductive to turn off FET23. Whereby, the resonant circuit of primary winding 21 and capacitor 24becomes active to make a resonance. At the end of one cycle of theresonance, the feedback voltage induced at feedback winding 25 reaches avoltage of turning on the gate of FET 23, thereby again making the FETconducive. The above operations are repeated to maintain the resonantvoltage or the electric pulse so as to oscillate the piezoelectricelement 110. The frequency of the resonant circuit is set variable inthe range of 1 MHz to 10 MHz.

Connected between the base of transistor 26 and resistor 27 is avariable resistor 30 of which value is varied in order to adjust atiming of turning on transistor 26 for regulating the resonantfrequency. It is noted in this connection that the resonant circuit iscontrolled by the control circuit 80 to give an intermittent oscillationhaving a rest period between adjacent pulse series Vp, as shown in FIG.5A.

The transformer T includes an auxiliary winding 91 which is cooperativewith a rectifier circuit that rectifies the output of auxiliary winding91 to constitute the monitoring circuit 90 which gives a monitoringoutput indicative of a condition of the ultrasound being applied to theuser's skin. The monitoring output Vx includes low frequency componentswhich are given as a result of moving the applicator head 100 and ofwhich frequency is lower than that of the ultrasonic vibration. Moreprecisely, the voltage appearing across auxiliary winding 91 includeslow frequency components originating from a variation in electricallyequivalent impedance of the combined vibration mass M upon contact withthe load and originating from rubbing sounds appearing in response tothe applicator head 100 moving across the skin of the user's skin, inaddition to high frequency components indicative of the ultrasoundvibrations. The monitoring output Vx is obtained by rectification ofvoltage appearing across auxiliary winding 91, and is fed to the loaddetecting circuit 40 and the motion detecting circuit 50 for making theload detection and the motion detection.

The load detecting circuit 40 has a comparator 41 which compares themonitoring output Vx from the monitoring circuit 90 with a referencelevel Vref. The monitoring output Vx has a waveform pattern as shown inFIG. 5B. When the output Vx becomes lower than the reference level Vref,the comparator 41 provides a H-level load detection signal SL to thecontrol circuit 80 as indicative of that the applicator head 100 is keptin the normal contact with the user's skin. When the load detectionsignal SL is not acknowledged continuously over a predetermined timeperiod, the control circuit 80 stops operating the driver circuit 20 ordisables the power supply 1. In this embodiment, the load detectionsignal SL is generated when the monitoring output Vx is lower than thereference level Vref in consideration of that the resonant voltage islowered by the presence of the load, i.e. the increased impedance of thecombined vibration mass M.

In addition, the output Vx indicative of the impedance of the combinedvibration mass M is fed also to the control circuit 80. When the outputVx is equal to the reference level Vref or greater, the control circuit80 operates to vary the output voltage of the power supply 1 a reverseproportion to the magnitude of the output Vx. That is, the combinedvibration mass M is held against the user's skin at a greater pressure,the control circuit 80 acts to lower the intensity of the ultrasoundbeing applied to the skin, and vice versa. With this result, theultrasound can be adjusted depending upon the pressure at which thecombined vibration mass M is held against the skin, thereby transmittingthe ultrasound at an optimal intensity for enhanced skin care effect.

It is possible that a resonant circuit of different configuration mayvary the impedance characteristic of the combined vibration mass M inorder to break the impedance matching with the resonant circuit, therebycausing the monitoring output to increase in the presence of the load.In this case, it is made to provide the load detection signal SL whenthe monitoring output Vx exceeds the reference level Vref. Also, it isequally possible to limit or reduce the ultrasound energy upon detectionof the no-load condition, and also to vary the ultrasound energydepending upon the magnitude of the monitoring output.

Further, the monitoring output Vx is fed through a capacitor 51 to themotion detecting circuit 50 in the form of an output Vx′, as shown inFIG. 5D. The motion detecting circuit 50 includes a low-pass filter 52and a judging circuit 53. The output Vx′ is removed of high frequencycomponents through the filter 52 to give a low frequency output VL freefrom the components not caused by the motion of the applicator head 100,as shown in FIG. 5E. Thus obtained low frequency output VL is fed to twocomparators 55 and 56 of the judging circuit 53 and comparedrespectively with individual thresholds TH1 and TH2 (TH1>TH2) to provideto the control circuit 80 a H-level motion detection signal SM (shown inFIG. 5F) over a period in which the output VL is higher than thethreshold TH1 or lower than the threshold TH2. TH1 and TH2 can beadjusted by variable resistors 57 and 58. The control circuit 80 countsthe time period of the H-level motion detection signal SM within apredetermined duration Tc (for example, 15 seconds) and determines thatthe applicator head 100 has moved suitably when the sum of the countedtimes within the duration Tc exceeds a predetermined reference.Otherwise, the control circuit 80 determines that no suitable motion hasbeen made and provides a limit signal of limiting the driver circuit 20.

The driver circuit 20 includes a transistor 84 which is connected inparallel with transistor 26 across gate-source path of FET 23 and whichis connected to the control circuit 80 through a photo-coupler 81. Thus,upon receiving the limit signal from the control circuit 80, thetransistor 84 is turned on to thereby turn off FET 23 for disabling thedriver circuit 20. Although the limit signal acts to stop the drivercircuit 20 in this embodiment, the present invention is not limited tothis feature and may be arranged to control the driver circuit 20 orpower supply 1 to reduce the ultrasonic vibration energy.

As shown in FIG. 6, the temperature sensing circuit 60 includes a firsttemperature sensing unit 61 and a second temperature sensing unit 62both receiving an output from a thermistor 15 for temperature sensing.First temperature sensing unit 61 has a temperature controller 65 towhich the output from thermistor 15 is fed through a resistor 63 and acapacitor 64. When the temperature sensed at thermistor 15 is found toexceed a predetermined reference temperature, the temperature controller65 issues a stop signal to the driver circuit 20 through a photo-coupler66. The photo-coupler 66 has a transistor 68 which is connected in abase-emitter path of the transistor 84, so that the stop signal causesthe transistor 84 to turn on for stopping the oscillation of the drivercircuit 20. A hysterics is given to the temperature control such that,after the temperature of the horn 120 sensed by thermistor 15 goes highabove the reference temperature, the driver circuit 20 is enabled toresume the oscillation only after the sensed temperature goes below atemperature level which is lower than the reference temperature. Whenthe sensed temperature goes below the temperature level, the temperaturecontroller 65 responds not to issue the stop signal, thereby resumingthe oscillation at the driver circuit 20. The second temperature sensingunit 62 includes a comparator 69 which operates to turn on a transistor160 when the temperature sensed at thermistor 15 exceeds a predeterminedreference, thereby turning on a transistor 163 of a photo-coupler 161and consequently disabling the power supply 1 connected to transistor163. The predetermined reference for the comparator 69 is set to behigher than the reference temperature of the temperature controller 65for stopping the ultrasonic oscillation as a safeguard in response tothe horn 120 being abnormally heated even if the temperature controller65 made of a microprocessor should fail to operate.

Operation of the ultrasound applying device is now explained withreference to FIG. 7. After turning on a power switch, pressing of astart button actuates the driver circuit 20, causing the piezoelectricelement 110 to start generating the ultrasound, and starting the timer.At this time, the temperature sensing is made for the horn 120 so thatwhen the first temperature sensing unit 61 sees the temperatureexceeding, for example, 45°, the display driving circuit 70 gives thetemperature warning that the horn 120 is over-heated, and causing thetimer as well as the driver circuit 20 to stop. When the sensedtemperature is found to be less than 45° C. at a step after starting thetimer, the load detection is available, and subsequently the motiondetection is available provided that the load detection signal is issuedas indicative of that the applicator head 100 is loaded. When no loaddetection signal is issued, a no-load warning is displayed for a limitedtime period of 40 seconds, for example, prompting the user to make theapplicator head 100 in contact with the skin. After elapse of 40 secondswith no load detection signal, a control is made to display a warning ofstopping the operation and stop the timer and the ultrasound generation.The motion detection is made in the presence of the load detectionsignal so that, when the motion detection signal is issued within, forexample, 15 seconds, a display of normal operation is made and acount-down instruction is given to the timer. After the elapse of apredetermined operation time, say, 10 minutes in this condition, thedriver circuit is stopped. When a pause button is pressed within 10minutes, the driver circuit is stopped but with the timer operatingcontinuously to count down. When a restart button is pressed within the10 minutes, the driver circuit resumes generating the ultrasound.

Although the above embodiment is so designed that the control circuitdisables the driver circuit when no load or no motion is detected, thepresent invention is not limited to this feature and is designed toreduce the ultrasound energy upon such detection.

Now referring to FIGS. 8 and 9, the details of the applicator head 100,i.e., the combination of the piezoelectric element 110 and the horn 120will be discussed. The piezoelectric element 110 is made of a ceramicand shaped into the circular disk having a uniform thickness and beingprovided on its upper and lower faces respectively with the upper andlower electrodes 111 and 112. The horn 120 is made of aluminum and isshaped into a circular disk having a surface area slightly larger thanthe piezoelectric element 110 and having a uniform thickness. Theelectric pulse from the driver circuit 20 is applied across theelectrodes 111 and 112 by way of lead wires 101 and 102 respectivelysoldered to the upper electrode 111 and the horn 120, as shown in FIG.9. The horn 120 is formed as an integrally part thereof with a tubularrim 130 which surrounds the horn 120. The rim 130 projects upwardly fromthe periphery of the horn 120 and is secured at its upper end to thehousing 10 to support the applicator head 100 to the housing. The upperend of the rim 130 fits snugly into a mouth 12 of the housing 10 with anelastic damper ring 132 interposed therebetween. The horn 120 has a flatmounting face 121 for carrying thereon the piezoelectric element 110 inan intimate contact relation, and a flat skin opposing face 122 forcontact with the skin through the fluid F spread on the skin S. Thepiezoelectric element 110 is secured to the horn 120 such that they areintegrated into the combined vibration mass M which resonates with theelectric pulse from the driver circuit 20 to generate the ultrasound tobe transmitted to the skin. A restrictor 140 in the form of a cavityextends between the horn 120 and the rim 130 in order to restrict theultrasound vibrations from propagating towards the rim 130, therebyconcentrating the ultrasound effectively to the user's skin, asindicated by arrows in FIG. 9. That is, the cavity 140 acts to isolatethe rim 130 substantially from the combined vibration mass M of thepiezoelectric element 110 and the horn 120 with regard to the ultrasoundvibrations. As a result of forming the cavity 140, a bridge 142 ofreduced thickness remains for connection of the horn 120 and the rim130. The reduced thickness (t) of the bridge 142 is selected to be otherthan an integral multiple of one-fourth of the wavelength of theultrasound (t≠n·λ/4, where n is an integer)) for effectively restrictingthe ultrasound vibrations from propagating towards the rim 130. As shownin FIGS. 10 and 11, the cavity 140 may be filled with a suitable medium144 for blocking the ultrasound vibrations, or may be finished withrounded edges 146. Further, as shown in FIG. 12, an additional cavity148 of different depth may be formed in a concentric relation to thecavity 140.

As shown in FIG. 13, the total thickness (T) of the combined vibrationmass M of the piezoelectric element 110 and the horn 120 is selected tobe half of wavelength (T=λ/2) of the ultrasound vibrating at a basicfrequency of, for example, 1 MHz so that the combined vibration mass Mcan resonate also at the frequencies that are integral multiples of thebasic frequency, for example, 2-fold, 3-fold, and 4-fold of the basicfrequency, while forming antinodes at the skin opposing face 122 of thehorn 120 and at the upper surface of the electrode 111, as schematicallyseen in the figure.

In order to transmit the ultrasound power effectively to the skin at aminimum loss and also to discriminate the normally loaded condition froman abnormally loaded or the unloaded condition when actuating thecombined vibration mass M around the resonant frequency, thepiezoelectric element 110 is designed to have a structure that restrainsvibrations at the center of the combined vibration mass M for reducingan undesired parasitic resonance which would otherwise makes the loaddetecting circuit 40 difficult to distinguish the normally loadedcondition from the unloaded or abnormally loaded condition. That is, asshown in FIG. 14, the parasitic resonance brings about fluctuationswhich are superimposed on the electrically equivalent impedance curvesas indicated by dotted lines with respect to varying frequency when thevibration mass M is under the unloaded condition or the abnormallyloaded condition. With this result, it becomes practically difficult todistinguish the normally loaded condition from the unloaded orabnormally loaded condition on the basis of the impedance of thevibration mass M in the vicinity of the resonant or antiresonantfrequencies. Consequently, it becomes hardly possible to extract thevarying impedance indicating the contacting pressure of the vibrationmass M within an admissible range as indicated by arrowed lines in thefigure in the vicinity of the resonant or antiresonant frequencies,failing to vary the intensity of the ultrasound in accordance with thepressure at which the combination mass M is held against the user'sskin, while the vibration mass M is in the normally loaded condition.

FIGS. 15 and 16 show one preferred structure for reducing the undesiredparasitic resonance to such an extent that the load detecting circuit 40can discriminate the normally loaded condition from the unloaded orabnormally loaded condition with reference to the electricallyequivalent impedance of the combined vibration mass M. In thisstructure, a center opening 114 is formed to extend the center of theupper electrode 111, the piezoelectric element 110, and the lowerelectrode 112 for restraining the vibrations at the center of thepiezoelectric element 110 and therefore the vibration mass M. With thisresult, the combined vibration mass M exhibits definite impedancecharacteristic curves in relation to the frequency under the unloaded orabnormally loaded condition, as indicated by dotted lines in FIG. 17,that can be well distinguished from the impedance curve that thevibration mass exhibits under the normally loaded condition, asindicated by solid line indicated in the same figure.

As apparent from FIG. 17, when subject to the unloaded or abnormallyloaded condition, the vibration mass M can give the electricallyequivalent impedance which are well distinctive from the impedance givenunder the normally loaded condition. By taking the advantage of thedistinction, the load detecting circuit 40 can discriminate theabnormally loaded or unloaded condition successfully simply bymonitoring the voltage reflecting the electrically equivalent impedanceof the vibration mass M, as explained hereinbefore with reference to themonitoring circuit 90. In this consequence, it becomes possible toextract the impedance varying with the contacting pressure of thevibration mass M within the admissible range as indicated by arrowedlines in the figure in the vicinity of the resonant or antiresonantfrequencies. Whereby, it can be made to vary the intensity of theultrasound in accordance with the pressure at which the combination massM is held against the user's skin, as long as the vibration mass M is inthe normally loaded condition.

In combination with the center opening 114, at least one of the upperand lower electrodes 111 and 112 may be shaped to have a diametersmaller than that of the piezoelectric element 110 to reduce thevibrations also at the periphery of the piezoelectric element andtherefore the combined vibration mass M for further reducing theparasitic resonance. The center opening 114 may be formed in at leaseone of the electrodes and the piezoelectric element, for example, asshown in FIGS. 18 and 19.

Further, as shown in FIGS. 20 and 21, the electrodes 111 and 112 may bedivided by diametrically extending slits 116 into four identicalsegments or sectors 117. The slits extend through the center of theelectrodes to leave the center and the diametrically extending bandportion of the piezoelectric element uncovered, thereby restraining thevibrations at the uncovered center and the band portions and thereforereducing the undesired parasitic resonance in order to realize theimpedance characteristic of FIG. 17 as well.

Alternatively, one or both of the electrodes 111 and 112 may be dividedinto two or eight segments 117, as shown in FIGS. 22 and 23 for the samepurpose.

Further, it is possible to give a slit 116A with closed ends also in thepiezoelectric element 110 and the electrodes 111 and 112, as shown inFIGS. 24 and 25, or to give a slit 116B open at its one end, or to giveparallel slits 116C, as shown in FIGS. 26 and 27, in at least one of theelectrodes and the piezoelectric element.

FIGS. 28 and 29 show a modification of the above embodiment in which acenter hole 134 is formed in the horn 120 in alignment with the centeropening 114 for further restraining the vibrations at the center of thecombined vibration mass M and therefore reducing the undesired parasiticresonance to a large extent. The combination mass M may be provided onlywith the center hole 134 in the horn 120, as shown in FIG. 30, In thisinstance, the center hole 134 may be in the form of a cavity, as shownin FIG. 31.

FIGS. 32 and 33 show an alternative structure in which an elastic member150 is secured on the center of the upper electrode 111 for absorbingand therefore restraining the vibrations at the center of thepiezoelectric element 110. The elastic member 150 is preferably made ofa silicone rubber. Instead of providing the elastic member 150, it isequally possible to give a weight on the center of the electrode 111 forrestraining the vibrations at the center of the piezoelectric element110 and therefore reducing the undesired parasitic resonance at thecenter of the combined vibration mass M. The weight is given by a solderbulk 160 or land used for electrical connection of the electrode 111 tothe lead wire 101 from the driver circuit 20.

It is confirmed that the structure, disclosed herein to reduce theundesired parasitic resonance by restraining at least the vibrations atthe center portion of the combined vibration mass M, is found effectiveto discriminate at least the unloaded condition from the loadedcondition in terms of the electrically equivalent impedance of thevibration mass M over the frequency range of 1 MHz to 10 MHz. In thisconnection, the device of the present invention may be configured not toreact with one of the abnormally loaded conditions shown in FIG. 2C, andtherefore allow the use without the fluid F.

It is noted in this connection that the individual structures shown withreference to FIGS. 15, 16, 18 to 35 can be suitably combined forreducing the undesired parasitic resonance. Further, where theelectrodes are concerned, it is possible that one of the electrodes canbe given the above structure for reducing the vibrations at the centerof the vibration mass, while leaving the other electrode to coversubstantially entirely the corresponding face of the piezoelectricelement 110.

1. An ultrasound applying skin care device comprising: a housingprovided with an applicator head which applies ultrasound to a user'sskin; and a driver circuit which gives an electric pulse for actuatingsaid applicator head to generate the ultrasound; said applicator headcomprising: a vibrator element generating the ultrasound, and a hornhaving a mounting face and a skin opposing face which is adapted in useto come into contact with the skin, said horn carrying said vibrator onsaid mounting face to transmit said ultrasound to the skin through saidskin opposing face, said vibrator element and said horn being integratedinto a combined vibration mass which resonates with the electric pulseof a resonant frequency from said driver circuit to generate theultrasound, said combined vibration mass giving a first electricallyequivalent impedance when it is normally loaded by contact with theskin, and gives a second electrically equivalent impedance when it isunloaded, a load detecting circuit which is connected to monitor whethersaid combined vibration mass gives the first or second electricallyequivalent impedance and provides a load detection signal only uponseeing said first electrically equivalent impedance, a control circuitwhich limits or stops the electric pulse when the load detection signalis not received within a predetermined time period, wherein saidcombined vibration mass has a structure that restrains vibrations at acenter portion of said combined vibration mass to reduce a parasiticresonance, thereby differentiating said first electrically equivalentimpedance from said second electrically equivalent impedance fordiscrimination therebetween, and wherein said vibrator element comprisesa piezoelectric element in the form of a circular disc having flat upperand lower end faces, and upper and lower electrodes respectivelydeposited on said upper and lower end faces, said electric pulse beingapplied across said upper and lower electrodes, and wherein said hornhas a center hole for restraining the vibrations at the center of saidcombined vibration mass.
 2. The ultrasound applying skin care device asset forth in claim 1, wherein at least one of said upper electrode, saidlower electrode, and said piezoelectric element has a center opening torestrain the vibrations at the center of said combined vibration mass.3. The ultrasound applying skin care device as set forth in claim 2,wherein at least one of said upper electrode and said electrode has adiameter smaller than that of said piezoelectric element to leave theperipheral portions of the corresponding end face of said piezoelectricelement uncovered.
 4. The ultrasound applying skin care device as setforth in claim 1, wherein each of said upper electrode, said lowerelectrode, and said piezoelectric element has a center opening torestrain the vibrations at the center of said combined vibration mass.5. The ultrasound applying skin care device as set forth in claim 1,wherein at least one of said upper and lower electrodes is divided by atleast one slit into a plurality of identical segments, said at least oneslit extending diametrically to leave the center and the diametricallyextending band portion of said piezoelectric element uncovered.
 6. Theultrasound applying skin care device as set forth in claim 1, wherein atleast one of said upper and lower electrodes has at least one slit thatuncovers the center portion of said piezoelectric element.
 7. Theultrasound applying skin care device as set forth in claim 1, whereinsaid upper electrode is covered on its center with an elastic memberabsorbing the vibrations at the center of said combined vibration mass.8. The ultrasound applying skin care device as set forth in claim 7,wherein said elastic member is a silicone rubber.
 9. The ultrasoundapplying skin care device as set forth in claim 1, wherein said upperelectrode of said piezoelectric element is covered on its center with asolder bulk for electrical connection of the upper electrode to a leadwire leading from said driver circuit, the solder bulk adding a weightto the center of the piezoelectric element.
 10. The ultrasound applyingskin care device as set forth in claim 1, wherein said horn is formed asan integral part thereof with a rim which surrounds said horn and isconnected to said housing, said horn and said rim defines therebetween arestrictor which restricts the ultrasound vibrations from propagatingtowards said rim.
 11. The ultrasound applying skin care device as setforth in claim 10 wherein said restrictor is defined by a cavity formedat the boundary between said horn and said rim.
 12. The ultrasoundapplying skin care device as set forth in claim 1, further including: amotion detecting circuit which monitors whether said combined vibrationmass is moving and provides a motion detection signal when saidvibration mass is so moving; said control circuit controlling saiddriver circuit to stop or limit said electric pulse when said loaddetection signal is not received within said predetermined time periodor when said motion detection signal is not continuous over a criticaltime duration even in the presence of said load detection signal beingdetected within said time period.
 13. The ultrasound applying skin caredevice as set forth in claim 1, wherein said control circuit receivessaid first electrically equivalent impedance in order to vary theintensity of the ultrasound generated at said vibrator element inaccordance with the magnitude of said first electrically equivalentimpedance.